A. Field of the Invention
The present invention relates to a zooming method and apparatus for a camera. More particularly, the present invention relates to a zooming apparatus and method for reducing backlash and focus error during a zooming operation.
B. Description of the Related Art
A variety of cameras having a zooming system include a zooming switch. The zooming switch further includes a telephoto angle switch and a wide angle switch which are operated by a user to move a zoom lens barrel during zooming. The telephoto angle switch operates the camera to zoom from a wide angle position to a telephoto angle position. The wide angle switch operates the camera to zoom from a telephoto angle position to a wide angle position.
During operation, the telephoto angle switch and the wide angle switch output a signal to a motor drive unit which drives the zoom lens barrel. An encoder then detects movement of the zoom lens barrel and generates a zoom position signal. Each zoom position is a discrete position that corresponds to a predetermined focal length. These zoom positions may be, for example, 38 mm, 45 mm, 55 mm, 60 mm, 65 mm, 75 mm, 80 mm, 90 mm, 100 mm, 105 mm, and 115 mm.
Thus, when a user stops the operation of the zooming switch, the zoom position stops at one of the above discrete zoom positions according to a zoom position signal received from the encoder. However, the movement of the zoom lens barrel will be different when zooming from a telephoto angle position to a wide angle position as compared to that when zooming from a wide angle position to a telephoto angle position. The difference in the movement of the zoom lens barrel is due to a backlash caused by the gears of the motor driving the zoom lens barrel and may, as a result, cause a focus error. A focus error may also occur due to inaccuracies of the encoder pattern. Therefore, the focal length (i.e. 90 mm) will be different when zooming, for example, from 80 mm to 90 mm as compared to when zooming from 100 mm to 90 mm.
A prior art driving method is disclosed in U.S. Pat. No. 5,280,317. The '317 patent discloses that when a telephoto angle switch is activated, the driving system drives a zoom motor in a forward direction and continues to do so even after the telephoto angle switch is deactivated. The driving system stops driving the zoom motor only after a predetermined period of time has elapsed. Similarly, when a wide angle switch is activated, the driving system drives a zoom motor in a backward direction and continues to do so until a predetermined period of time has elapsed after the wide angle switch has been deactivated. The driving system then drives the zoom motor in a forward direction until an encoder signal of a destination zoom position is received. FIG. 5 is a diagram illustrating the zooming operation from a telephoto angle position to a wide angle position using the system disclosed by the '317 patent.
For example, when zooming from the 100 mm zoom position to the 80 mm zoom position, the driving system drives the zoom motor in a backward direction from the 100 mm position to the 75 mm position in order to read from the encoder a border between the 80 mm and 75 mm positions. The driving system then reverses the zoom motor in a forward direction to move the zoom lens barrel to the 80 mm zoom position. FIG. 6 is a table illustrating the zoom start and stop positions corresponding to a plurality of focal lengths of the prior art zooming system.
The method of the '317, however, suffers from the fact that it cannot eliminate a focus error due to the inaccuracies of the encoder pattern. A second problem is that the driving of the motor in both the forward and backward directions when changing zoom positions causes visual discomfort on the user. Therefore, there is a need for a user-friendly zooming system which can eliminate the inaccuracies of the encoder pattern.